Wearing a singlet I put down is simplest (7)
I believe the answer is:
naivest
'simplest' is the definition.
('naive' can be similar in meaning to 'simple')
'wearing a singlet i put down' is the wordplay.
'wearing a singlet' becomes 'naive' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'i' becomes 'a'.
'put' becomes 'sat' (both can mean positioned or placed).
'down' is a deletion indicator ('down' can mean missing).
'sat' with 'a' taken out is 'st'.
'naive'+'st'='NAIVEST'
'is' is the link.
Can you help me to learn more?
(Other definitions for naivest that I've seen before include "Most unsophisticated" , "Least sophisticated" , "Most simple" , "far from the most wise?" , "very green" .)